The present invention, as well as the pertinent prior art, is conveniently discussed in the context of a multi channel magnetic tape apparatus. Bubble memory, optical storage, point-to-point communication and the like are not excluded from the practice of this invention, but for purposes of specificity and example, reference will frequently be made to multi-track magnetic tape apparatus.
Multi-channel apparatus of the type of interest is most frequently employed to develop information serial by bit in a plurality of channels, to realize byte parallel data for impression on the media and recovery therefrom at a data receiver. It is immediately clear that the mutual synchronization among the data streams on the respective channels is critical and the synchronization of these data streams is the subject of the present invention. It is useful to observe at the outset that the impression on the channel and recovery of the time scale therefrom is not the subject addressed by the present invention: rather it is the relative location on that time scale of correlated events which is sought when the correlation between the several data channels has been disturbed. The nature of such disturbances is determined by the apparatus. For the example of multi-track magnetic tape storage and recovery there are a number of flaws of mechanical origin which contribute skew between the several tracks. It is beyond the scope of this work to address specific causes of skew, drop-outs and like disturbances which disturb the synchrony of the several channels. All such errors will be referenced as mis-alignment or skew regardless of the precise character or nature of the problem.
The conventional practice for such apparatus is to insert synchronization information on each data channel. In order to accomplish this insertion, there must be provided for each channel a buffer accepting user data at a fixed rate and emitting the same user data at an irregular (higher) rate providing gaps in the emitted data stream to accomodate the synchronization information. (No distinction is drawn in this work between data, e.g., user data, and redundancy annexed to such data for error correction purposes.) The synchronization information usually takes the form of a unique bit pattern. The buffer is, at least, of sufficient length to accomodate this gap. Skew, bit slippage, channel noise and other parasitic effects may disturb the information whereby the real-time correlation of the several data streams is degraded. A conventional practice for recovery of synchrony proceeds by recovery of the time scale through observation of the several data streams. For each channel, the bit stream is searched for the synch pattern and the several channels are each labeled by a value of skew or misalignment. The respective data streams are relatively delayed or advanced in accord with the respective skew values to bring the channels back into time alignment.
In a system as described, one observes that bandwidth is consumed in each data channel by the respective synchronization information. Moreover, it frequently occurs that there are overriding constraints on power consumption, weight, or other specifications, notably occuring in space-borne telemetry, which suggest that savings may be realized in accomplishing requisite synchronization.
In the prior art there is known an interleaver/de-interleaver synchronizer apparatus and method for acquiring synchrony on a single data stream, eg. transmitter-receiver synchrony, derived from a coherent reinforcement principle. This arrangement is implemented in a plurality of counters each corresponding to an identifiable discrete bit interval (phase) of an interleaver period. These counters record the relative observed probability for any particular phase within the data stream to be the synch phase. To each phase there is associated a counter which is either incremented (or decremented) in accord with a detected match (or non-match) between a windowed portion of the data stream and a predetermined synch bit pattern. Overflow of a counter indicates in a probablistic sense the synch phase, establishing the synchronous condition. This system is described in U.S. Pat. No. 4,633,486, assigned to Cyclotomics, Inc.